Food chains and food webs

Organisms are connected by feeding relationships: when one organism eats another, food and energy are transferred between them.

Producers, usually green plants or algae, use sunlight to make organic material by photosynthesis. Consumers obtain food by eating other organisms. A primary consumer eats a producer; a secondary consumer eats a primary consumer. Higher-level consumers eat other consumers.

Food chains: one feeding pathway

A food chain shows one pathway along which food and energy are transferred. Each arrow points from the organism being eaten towards the organism that eats it.

In a small UK freshwater pond, algae can be eaten by water fleas, which can be eaten by small fish:

The algae are the producer, the water fleas are primary consumers, and the small fish are secondary consumers in this chain.

Food webs: connected feeding pathways

A food web links several food chains to show the feeding relationships within a community. It gives a fuller picture than a single chain because an organism may eat several species and may itself be eaten by more than one species.

In the pond, consumers other than water fleas can feed on algae, and small fish can eat prey other than water fleas. These connections mean that a change in one population can affect both the organisms it eats and the predators that eat it.

For example, if the water flea population falls, less algae may be eaten and small fish may have less food. The effects can extend beyond the algae–water flea–fish chain because other consumers and prey are connected to it in the food web.

Photosynthetic production of biomass

Photosynthetic organisms use light energy to make glucose from carbon dioxide and water. Oxygen is released during this process, called photosynthesis.

Glucose provides material for making other organic molecules. For example, plants use glucose to make cellulose for cell walls and starch for storage. These substances contribute to the organism’s biomass: the mass of its living material.

Green plants and algae are producers because they make their own organic material rather than obtaining it by eating other organisms. When a consumer eats a producer, it obtains organic material that can be used to build its own biomass. Through feeding relationships, producers therefore support the biomass of other organisms.

Quadrat sampling

A quadrat is a frame that marks out a known area. Organisms recorded inside it provide a sample that can be used to estimate the abundance of a species across a larger study area.

Organisms may be concentrated in some places and scarce in others. A single quadrat may therefore give a misleading picture. Using several quadrats at randomly selected positions reduces the risk of choosing only places where the species is especially common or rare. Positions can be selected by marking out a coordinate grid over the study area and choosing coordinates at random.

Recording abundance in quadrats

The method of recording depends on the organisms:

• Count: Record the number of individuals of the species inside each quadrat. This works when individuals can be distinguished and counted.
• Percentage cover: Estimate the percentage of the quadrat’s area covered by the species. This is useful when individuals are difficult to distinguish, such as plants growing close together.

Percentage cover describes how much area a species covers; it does not give a count of individuals.

Estimating a population from quadrat counts

First, find the mean count per quadrat by adding the counts and dividing by the number of quadrats. Then scale this mean up by the number of quadrat-sized areas in the whole study area:

Use the same units for both areas. For example, if the mean count is 5 plants per quadrat, the quadrat area is $$0.25,\text{m}^2$$ and the study area is $$100,\text{m}^2$$:

This is an estimate, not an exact count. The calculation uses the sample mean as an estimate of the mean throughout the study area; it does not mean that every quadrat-sized area contains the same number of plants. Taking enough randomly placed quadrats to represent the variation across the area improves the reliability of the estimate.

Transect sampling

A transect is a line laid across a habitat. Organisms are recorded along it so that their distribution can be compared at different positions. For example, a transect might run from a dry part of a habitat towards a wetter part.

Line and belt transects

In a line transect, the organisms touching the line are recorded at known positions along it. This shows which organisms occur at different points on the route.

In a belt transect, quadrats are placed along the line. A quadrat is a frame used to mark out an area for sampling. The species present, or their abundance—how many organisms of each species are found—are recorded in each quadrat. Quadrats can be placed continuously, next to one another along the route, or at set intervals.

Comparing positions along a transect

The records show how the community—the populations of different species living in the area—changes across the habitat. A transect can reveal changes along an environmental gradient, where a condition such as moisture changes gradually across the area.

Sampling positions must be recorded consistently, for example by noting their distance from the start of the transect. In a belt transect, using the same quadrat size and recording method at each position makes the results more suitable for comparison.

Measures of central tendency

A measure of central tendency gives a single value that represents the centre or a typical observation in a data set. The mean, median and mode describe this in different ways, so they do not always give the same answer.

These measures can be used to summarise repeated observations of organism abundance, such as counts of organisms in several sampled areas.

Mean

The arithmetic mean is found by adding all the observations and dividing the total by the number of observations.

Median

The median is the middle value when the observations are arranged in numerical order. If there are two middle values, calculate their mean to find the median. For example, in $$2, 3, 4, 8$$, the middle values are $$3$$ and $$4$$, so the median is $$3.5$$.

Mode

The mode is the value that occurs most frequently. A data set can have more than one mode if values share the highest frequency. It can also have no mode, for example when every value occurs only once.

Summarising abundance observations

Suppose five sampled areas contain $$3, 8, 2, 4$$ and $$3$$ organisms. To find the median, first put the counts in order: $$2, 3, 3, 4, 8$$.

• Mean: The total is $$20$$, so the mean is $$20 \div 5 = 4$$ organisms.
• Median: The middle count is $$3$$ organisms.
• Mode: The most frequent count is $$3$$ organisms; it occurs twice.

Plotting abundance data

A graph makes it easier to see patterns in abundance and compare values. Choose the graph type according to what abundance is being compared against.

Choosing a graph for abundance

Use a bar chart to compare separate categories, such as different habitats. Put the categories on the horizontal axis and abundance on the vertical axis. Draw bars of equal width, with gaps between them; the height of each bar represents the abundance for that category.

Use a line graph to show how abundance changes with a continuous variable, such as time or distance. Put the continuous variable on the horizontal axis and abundance on the vertical axis. Plot each pair of values as a point, then connect the points with a line.

Labelling and plotting abundance

Label each axis with the quantity it shows and include units where appropriate. For example, distance might be labelled in metres, while abundance might be labelled as the number of organisms counted.

Choose scales with consistent intervals that cover the full range of the data and make effective use of the graph area. Read both scales carefully when plotting each value: draw each bar to the correct height or place each point at the position given by its two values.

Predator–prey population cycles

A predator is a consumer that kills and eats another animal. The animal eaten is its prey. In a stable community, their populations can rise and fall in repeated cycles rather than staying at fixed sizes.

How the populations affect each other

When the prey population increases, more food is available for predators. More predators may survive and reproduce, so the predator population can increase. This increase happens after a delay, not as soon as prey numbers rise.

As predator numbers increase, more prey are eaten. The prey population may then fall. With fewer prey available as food, predator numbers may also fall after a delay. Fewer predators means less predation, allowing the prey population to recover and the cycle to repeat.

Reading predator–prey population graphs

On a population graph, the horizontal axis usually shows time and the vertical axis shows population size. Each line shows how one population changes over time. A peak is a high point in a population; a trough is a low point.

The predator peaks typically follow the prey peaks: prey numbers rise first, and predator numbers rise later as food becomes more available. Similarly, predator numbers may reach a trough after prey numbers have fallen. Comparing the timing of the two lines helps show the delay between a change in the prey population and the response of the predator population.